This invention relates to a system for use in creating and carry out subsequent operations in a well and, in particular, to a system which allows access to a well bore, either subsea or at the surface, whilst ensuring that the well pressure at the wellhead is controlled.
When drilling a well, the single mechanical safety barrier is the BOP stack connected to the wellhead. The BOP ram package contains the high pressure rams and usually the medium pressure annular. For subsea the BOP lower riser package, connected to the upper end of the BOP ram package is arranged to be disconnected from the BOP ram package, for example, in the event of an emergency to release the riser and vessel.
The need to drill at either a “low pressure” or for kick control is required when entering a permeable low pressure formation which requires to be drilled with a specific mud. Most top sections of a hole can be drilled with a standard mud and in a method which follows conventional practices.
Currently, wells are drilled from the surface with mud columns and the necessary monitoring is carried out from, or at, the drill floor. When drilling a well, the mud weight is a critical factor for three main reasons. Firstly, the pressure differential on the kick/fracture gradient which gives a drilling pressure margin between the pore and fracture pressures is critical. Secondly, the formation pressure versus the mud pressure (hydrostatic head and circulation pressure) affects the rate of penetration. Finally, the kick control pressures need to be controlled in order to manage a well kick and to maintain the ability to circulate the gas out of the well within the well design limits.
Drilling a production well with a balanced or overbalanced mud weight can clog open formations causing severe permanent permeability damage, i.e. the hydrocarbon flow rate from the well is drastically reduced. A solution is to drill the formation with a mud column which has a lower hydrostatic head pressure than the reservoir pressure. This is generally known as under balanced drilling. Muds that can achieve this could be water based, oil based, foam gas or a combination.
For surface BOP operations, the use of certain preferred muds could result in the mud pressure in the BOP being above atmospheric pressure.
A means of sealing around the drilling string, whilst at the same time allowing the tubular equipment to move in or out of the well is necessary. In known configurations, downstream of a surface BOP, the well annulus flow has to be choked back prior to entering a low pressure separator in order to remove any entrained gas in the return mud flow. The mud at atmospheric pressure then passes over shale shakers to remove the cuttings prior to entering the mud pumps. For subsea BOP operations, the density of the mud column means it is necessary to operate through the surface BOP with the wellhead pressure being less than the mud hydrostatic head to the surface.
Typically, underbalanced drilling has been limited to coil tubing drilling because of its simplicity, i.e. the coil tubing is not rotated, has no tool joints and coil tubing BOP's are designed for tubing to be stripped in and out of the hole under pressure. An important criteria for an underbalanced well is that, at no time in its life, must the formation be subjected to over pressure. As the formation has no protection from excessive pressure, considerable damage will occur very quickly. This applies in all aspects of accessing the well, for example, during drilling the well, casing operations, running the completion, testing and workovers.
One of the prime advantages of underbalanced drilling is that production from the open and free formations can be up to 50% higher than if the well had been drilled in a conventional manner. Typically, under balanced drilling is only performed on land wells, as opposed to subsea wells, due to the complexity of the operation and the considerable size of the hardware required. For example, it is necessary to have access to the BOP, the chokes, the separators, the pumps, and it must be possible to control the system carefully.
In deep water (i.e. 1000 metres or more), some reservoirs are not subjected to the overburden pressure of the ground above, which would be experienced on land or shallow water location wells. This means these reservoirs have a low overburden pressure relative to the hydrostatic head of water. Therefore, a small increase in pressure can result in fracturing the formation, causing an underground blow out. A small decrease in mud pressure will result in a well kick. Accordingly, there is a narrow operating pressure margin in between the formation pore and fracture pressures which limits the distances which can be drilled in one stage especially in a horizontal section. In order to drain a reservoir fully, it is typically necessary for more wells to be provided and these wells need to be evenly spaced across the field to ensure that all regions of the field are accessed. Accordingly, developments of this sort are extremely costly.
When drilling in deeper waters, the operator of a well must remain within the safe drilling limits in order to control the well completely and so that it is possible to handle unplanned scenarios which include well kick operations such as kick detection relative to the large mud circulating volume, movement of a floating vessel that alters the mud circulation volume, the gelling effect of muds which increases the friction and therefore the circulating pressure, the friction caused by the limited size and length of the choke and kill lines, operating in the narrow mud weight window, and then attempting to control a kick on the surface.
U.S. Pat. No. 6,276,455 shows a horizontal subsea separation system which is piped off the base of the riser unit to a sea bed gravity separation unit, and with an independent riser for mud returns. The system operates at the ambient water pressure and therefore would not support any mud in the drilling riser bore.